Shake the bottle well before adding it to the test tube,
Rinse test tubes with the same aquarium water your testing before filling.
Make sure you cap and shake between drops.
The way I judge it is:
If it turns slightly green for example on 3, then darker on 4, I write down gh 3-4.
You can look down through the tube to get a better reading.
Count ALL drops, out loud as I add them helps for me as I have a frail memory lol
The cheaper kits like API can be a bit of a hassle, so don't worry too much, for example on the nitrate test the 5, 10, and 20ppm are all the same shade of orange lol. Apart from this the rest of the API kits like gh etc. seem to work fine.